Disk unit and process for assembling transmission mechanism used for driving head carriage

ABSTRACT

Coil springs 96 and 97 are incorporated in a driving-side transmission mechanism 61 so as to absorb backlashes between a rack 55 and a drive gear 59a. Since the respective backlashes between the drive gear 59a and the rack 55 are absorbed by way of the coil springs 96 and 97, merely phase matching at the sole driving-side transmission mechanism 61 is required as phase matching work of toothed portions of gears during assembly, thereby simplifying the assembling task and shortening assembly time. Additionally, since a coil spring is not included in the rack 55, the rack can be designed to be small-sized in its width direction, thereby minimizing the moment of a force about the meshed engagement point, acting on an optical head carriage 53, and reducing transfer loss in the head.

FIELD OF THE INVENTION

The present invention relates to a disk unit which is optimally used fora compact disk player, a CD-ROM drive or the like, and to a process forassembling a transmission mechanism used for driving a head carriage.

BACKGROUND ART

An optical disk unit is traditionally equipped with an optical pick-updevice that is movable frontwards and backwards by means of a drivemotor and a disk table which is rotatably driven by means of a spindlemotor. An optical disk is placed on the disk table for rotation togethertherewith. As is generally known, the optical disk unit operates to readand reproduce a predetermined information signal recorded in a recordingtrack through a record-tracking operation according to which thepredetermined information signal is recorded by vertically emitting alaser beam generated from the optical pick-up device to the optical diskfrom its bottom, while transferring the optical pick-up device in aradial direction of the optical disk.

In such optical disk units, since the high-density information signalrecorded in the optical disk is read while transferring the opticalpick-up device from the inner periphery of the optical disk to the outerperiphery, there is a possibility that tracking error in the recordingtrack will result from an error in the transfer of the optical pick-updevice. As a result the information signal is not read and thus a goodreproduction cannot be made owing to the lack in reproducing signals.

For the reasons indicated above, the driving-force transmission gearmechanism which is provided for transferring the optical disk unit, isconventionally equipped with a backlash eliminating mechanism, foreliminating backlash between a meshing pair of gears and for enhancingreliability in the transfer of the optical pick-up device.

One such conventional optical disk unit will be hereinbelow explained inaccordance with FIG. 39.

In this drawing, reference numeral 1 denotes a mechanical deck which isformed with a through window la being substantially rectangular from theplan view and opening in a vertical direction of the deck and a cut-out1b being substantially semi-circular in the plan view and integrallyformed with the through window 1a. The mechanical deck is disposed in abody (not shown) of an optical disk player.

Reference numeral 2 denotes a spindle motor which has an output shaftpenetrating the cut-out 1b and is fixed on the reverse side (undersurface) of the mechanical deck 1.

Reference numeral 3 denotes a disk table, on which the disk is placed,is fixed on the top of the output shaft 2a and has a driven connectionwith the spindle motor 2 for co-rotation therewith.

Reference numerals 4 and 5 denote a pair of right and left guide shaftsrespectively extending in the advancing and retreating directions of anoptical head carriage as set out below. The guide shafts are arranged inparallel with each other in the body (not shown) of the player and fixedat the perimeter of the window 1a on the reverse side of the deck.

Reference numeral 6 denotes the optical head carriage which is providedfor transferring an optical pick-up device 7 in the front and reardirections and slidably mounted on the guide shafts 4 and 5 throughbushings 8.

Reference numeral 9 denotes a rack mechanism which is provided fortransmitting a driving force produced by a motor. The rack mechanismconsists of a first rack 10 extending frontwards and rearwards and fixedon one side of the optical head carriage 6, a second rack 12 extendingfrontwards and rearwards and slidably mounted on the upper surface ofthe first rack 10 through a spacer 11, and a compression coil spring 13operably disposed between the first and second racks 10 and 12 forspring-loading them and serving as a backlash eliminating mechanism byproducing the spring bias so that the two racks are spaced apart fromeach other.

Reference numeral 14 denotes a motor which is fixed on the upper surfaceof the mechanical deck 1 at one side of the latter for driving the headcarriage. A motor gear 15 is fixedly attached to an output shaft 14a ofthe motor 14 in such a manner as to be placed at the reverse side of themechanical deck 1.

Reference numeral 16 denotes a drive gear rotated by driving the motor14. The drive gear is rotatably mounted on the reverse side of themechanical deck 1 by way of a support shaft 17.

Reference numeral 18 denotes an intermediate driving-force transmissiongear mechanism is provided for transmitting a driving force produced bythe motor 14 to the optical head carriage 6. The intermediatetransmission gear mechanism is mechanically linked to both the rackmechanism 9 and the drive gear 16 and consists of a driving-sidetransmission gear mechanism 19 and a driven-side transmission gearmechanism 20.

Of these transmission gear mechanisms 19 and 20 constructing theintermediate transmission gear mechanism 18, the driving-sidetransmission gear mechanism 19 consists of two gears 21 and 22 bothbeing in meshed engagement with the drive gear 16 and being rotatablydisposed, and a compression coil spring 23 serving as a backlasheliminating mechanism which acts to bias the respective gears 21 and 22in their opposite peripheral directions.

Of these gears 21 and 22, the gear 21 is integrally formed with anupwardly extending small gear 24 at its central portion.

On the other hand, the driven-side transmission gear mechanism 20 of theintermediate transmission gear mechanism 18 consists of two gears 25 and26 both being in meshed engagement with the small gear 24 and beingrotatably disposed.

Additionally, the gears 25 and 26 are in meshed engagement with thefirst and second racks 10 and 12, respectively.

In the optical disk unit as set forth above, the driving force producedby the motor 14 is transmitted from the drive gear 16 through both thegears 21 and 22 of the driving-side transmission gear mechanism 19 andthrough both the gears 25 and 26 of the driven-side transmission gearmechanism 20 to the first and second racks 10 and 12 of the rackmechanism 9, with the result that the optical head carriage 6 can bemoved along both the guide shafts 4 and 5 in the front and reardirections.

At this time, backlash between the drive gear 16 and the gear 21 andbacklash between the drive gear and the gear 22 are both absorbed bymeans of the compression coil spring 23, while backlash between the gear25 and the rack 10 and backlash between the gear 26 and the rack 12 areboth absorbed by means of the compression spring 13. Additionally, therespective backlash between the small gear 24 and each of the gears 25and 26 is absorbed by means of both the compression coil springs 13 and23.

In the prior art optical disk units, since the two backlash eliminatingmechanisms (the compression coil springs 13 and 23) are respectivelyprovided in the rack mechanism 9 and the driving-side transmission gearmechanism 19, the prior art optical disk unit requires phase matching atrespective toothed portions of the rack mechanism 9 and the driving-sidetransmission gear mechanism 19. This results in a troublesome assemblytask when totally assembling the respective mechanisms. Additionally, ittakes a long time for this assembly task.

Furthermore, since the compression coil spring 13 is incorporated in therack mechanism 9, the rack mechanism 9 is so designed as to have anundesirably large dimension in its width direction. The above-notedlarger dimension results in a long distance from the guide shaft 5 tothe engaging point of the rack mechanism 9 and the driven-sidetransmission gear mechanism 20. As a result, when transferring theoptical pick-up device 7, the moment of a force about the above-notedengaging point with regard to the optical head carriage 6 tends toincrease, thereby increasing transfer loss in the head carriage.

SUMMARY OF THE INVENTION

It is, therefore, in view of the above disadvantages, an object of thepresent invention to provide a disk unit which is easier to assemble,has reduced assembling time, and has reduced transfer loss of its headcarriage.

It is another object of the present invention to provide a disk unitwhich is capable of performing a smooth transfer of a head carriage.

An optical disk unit according to the present invention includes firstand second backlash eliminating mechanisms respectively provided in adriving-side transmission gear mechanism for eliminating backlashbetween a rack and a drive gear.

Therefore, in case of the present invention, backlash between the rackand the drive gear is eliminated by means of the first and secondbacklash eliminating mechanisms incorporated in the driving-sidetransmission gear mechanism corresponding to an intermediatedriving-force transmission gear mechanism. The disk unit made inaccordance with this invention only requires phase matching at the soledriving-side transmission gear mechanism in order to achieve phasematching at the toothed portions of the driving-side transmission gearmechanism with the backlash eliminating mechanism.

Additionally, since the backlash eliminating mechanism is not includedin the rack, the dimension of the rack in its width direction can bedecreased. Thus, when transferring a head, the moment of a force aboutthe engaging point (which moment acts on the head carriage) becomessmall.

Another embodiment of disk unit according to the invention may furtherinclude a head carriage equipped with a support shaft which shaft isarranged in parallel with axes of respective driving-force transmissiongears for rotatably supporting the rack.

Therefore, in the unit of the invention, since the rack is rotatableabout the support shaft, the load acting on the rack through theintermediate transmission gear mechanism is dispersed.

A disk unit according to the invention may further include openingswhich are respectively provided in a first driving-side transmissiongear, a second driving-side transmission gear, and a lever, for thepurpose of phase matching at the toothed portions.

Therefore, in the unit, phase-matching between toothed portions of therespective driving-side transmission gears of the driving-sidetransmission gear mechanism is made by matching all of the throughopenings of the first driving-side transmission gear, the seconddriving-side transmission gear and the lever.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view illustrating a transmissionmechanism for use in a head carriage driving device incorporated in adisk unit according to the present invention.

FIG. 2 is a plan view illustrating the transmission mechanism drivingthe head carriage incorporated in the disk unit according to theinvention.

FIG. 3 is a cross-sectional view illustrating the transmission mechanismdriving the head carriage of the disk unit according to the invention.

FIG. 4 is an exploded perspective view illustrating the head carriageand the transmission mechanism driving the head carriage of the diskunit according to the invention.

FIG. 5 is a bottom view illustrating the head carriage and thetransmission mechanism driving the head carriage of the disk unitaccording to the invention.

FIG. 6 is a cross-sectional view illustrating a drive gear of adrive-gear mechanism in the disk unit of the invention.

FIG. 7 is a cross-sectional view illustrating an assembling state ofgears of a driving-side transmission gear mechanism in the disk unit ofthe invention.

FIGS. 8(A) and 8(B) are a plan view illustrating a first driving-sidetransmission gear of the driving-side transmission gear mechanism of thedisk unit of the invention and a cross-sectional view taken along theline B--B shown in FIG. 8(A), respectively.

FIGS. 9(A), 9(B) and 9(C) are cross-sectional views respectively takenalong the line a--a, along the line b--b, and along the line c--c, eachline shown in FIG. 8(A).

FIGS. 10(A) and 10(B) are a plan view illustrating a second driving-sidetransmission gear of the driving-side transmission gear mechanism of thedisk unit of the invention and a cross-sectional view taken along theline B--B shown in FIG. 10(A), respectively.

FIGS. 11(A), 11(B) and 11(C) are cross-sectional views respectivelytaken along the line a--a, along the line b--b, and along the line c--c,each line shown in FIG. 1O(A).

FIGS. 12(A), 12(B) and 12(C) are a plan view illustrating a thirddriving-side transmission gear of the driving-side transmission gearmechanism of the disk unit of the invention, a cross-sectional viewtaken along the line B--B shown in FIG. 12(A), and a view in thedirection of the arrow C in FIG. 12(B), respectively.

FIGS. 13(A) and 13(B) are a cross-sectional view illustrating a lever ofthe driving-side transmission gear mechanism of the disk unit of theinvention and a plan view illustrating the same, respectively.

FIGS. 14(A) and 14(B) are a plan view illustrating a first driven-sidetransmission gear of a driven-side transmission gear mechanism of thedisk unit of the invention and a cross-sectional view taken along theline B--B shown in FIG. 14(A), respectively.

FIGS. 15(A) and 15(B) are a plan view illustrating a second driven-sidetransmission gear of the driven-side transmission gear mechanism of thedisk unit of the invention and a cross-sectional view taken along theline B--B shown in FIG. 15(A), respectively.

FIGS. 16(A), 16(B) and 16(C) are a plan view illustrating a rack whichwill be acted upon by a driving force produced by a head-carriagedriving motor in the disk unit of the invention, a cross-sectional viewtaken along the line B--B shown in FIG. 16(A), and a bottom view of therack, respectively.

FIGS. 17(A) and 17(B) are a left-hand side view illustrating the rackwhich will be acted upon by the driving force produced by thehead-carriage driving motor in the disk unit of the invention and aright-hand side view illustrating the same, respectively.

FIG. 18 is a plan view illustrating a snap ring associated with the rackwhich will be acted upon by the driving force produced by thehead-carriage driving motor in the disk unit of the invention.

FIGS. 19(A) and 19(B) are a plan view illustrating a thrust receivingplate associated with both the intermediate transmission gear mechanismand the drive-gear mechanism in the disk unit of the invention and anelevational view illustrating the same, respectively.

FIG. 20 is a cross-sectional view illustrating a meshing state among therack and the respective transmission gears of the driven-sidetransmission gear mechanism in the disk of the invention.

FIG. 21 is a cross-sectional view illustrating a meshing state among thedrive gears of the drive-gear mechanism and the transmission gears ofthe driving-side transmission gear mechanism in the disk unit of theinvention.

FIGS. 22(a) and 22(b) are explanatory views, respectively indicating astate of transmission of the driving force produced by the head-carriagedriving motor and a state of application of spring-bias in the disk unitof the invention.

FIG. 23 is a plan view illustrating a state in which the head carriageof the disk unit of the invention is transferred in an inner peripheralposition of the disk.

FIG. 24 is a plan view illustrating a state in which the head carriageof the disk unit of the invention is transferred in an outer peripheralposition of the disk.

FIG. 25 is a plan view indicating an emergency ejecting operation in thedisk unit of the invention.

FIG. 26 is a plan view indicating a state of completion of loading of adisk tray in the disk unit of the invention.

FIG. 27 is a plan view indicating a state of completion of chucking ofthe disk in the disk unit of the invention.

FIG. 28 is a perspective view illustrating the disk tray of the diskunit of the invention, partly cut-out.

FIG. 29 is a plan view indicating relative position relations among arack of the disk tray, a guide groove, pinions, and a guide pin in thedisk unit of the invention.

FIG. 30 is a cross-sectional view taken along the line H--H shown inFIG. 28.

FIG. 31 is an exploded perspective view illustrating a disk-tray drivingmechanism in the disk unit of the invention.

FIGS. 32(A) and 32(B) are views in the direction of the arrow I--I inFIG. 31, indicating a rotational movement of a holder driving lever inthe disk unit of the invention.

FIG. 33 is a view in the direction of the arrow J--J in FIG. 31,indicating the emergency ejecting operation in the disk unit of theinvention.

FIG. 34 is a plan view illustrating a state in which the upper cover ofthe disk unit of the invention is removed.

FIG. 35 is a cross-sectional view taken along the line K--K in FIG. 34,indicating a disk-chucking released state in a disk chucking mechanismof the disk unit of the invention.

FIG. 36 is a cross-sectional view taken along the line K--K in FIG. 34,indicating a disk chucking state in the disk chucking mechanism of thedisk unit of the invention.

FIG. 37 is a perspective view illustrating a state of completion ofejection of the disk tray in the disk unit of the invention.

FIG. 38 is a perspective view illustrating a state of completion ofloading of the disk tray in the disk unit of the invention.

FIG. 39 is a perspective view illustrating a head carriage and atransmission mechanism for driving the head carriage in the prior artdisk unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The construction of the present invention will be hereinbelow describedin detail on the basis of the embodiment shown in FIGS. 1 to 38.

In these drawings, denoted by reference numeral 100 is a body of anoptical disk unit, which body has a front opening. The body isconstructed by a substantially box-shaped casing 101 having astainless-steel upper cover 101a and a stainless-steel lower cover 101band a synthetic-resin front panel 102 fitted into the front opening ofthe casing 101 and having an opening 102a through which the disk traygoes in and out (See FIGS. 35 and 36).

The body 100 accommodates therein a printed circuit board 103 whichboard is positioned above the lower cover 101b of the two covers 101aand 101b.

The front panel 102 of the body 100 is formed with an emergency ejecthole 102b penetrating the outside of the panel to the inside. In thevicinity of the emergency eject hole 102b, an eject button 140 isinserted into the panel so that a portion of the button is projectedoutside of the panel (See FIG. 37).

Reference numeral 104 denotes a synthetic-resin disk tray which tray isconstructed by a disk placing plate 105 and a front plate 106. The diskplacing plate has a concavity 105a being substantially circle from theplan view and accommodating therein an optical disk D, and has anopening 105b extending at the bottom area of the concavity 105a in thefront and rear directions (directions indicated by the arrows b and a).The front plate is integrally formed with the disk placing plate 105 atthe front end of the latter, for opening and closing the above-mentionedopening 102a. The disk tray advances into and retreats from the casing101.

The disk placing plate 105 of the disk tray 104 is integrally formedwith projected portions 105c flanged from both sides of the tray toguide the tray and with additional projected portions 105d provided forrestricting the movement of the tray (See FIGS. 28 and 30).

During loading of the disk, upon the front plate 106 is lightly pushedin the direction indicated by the arrow a after the optical disk D hasbeen placed onto the concavity 105a of the disk placing plate 105 asshown in FIG. 37, a loading switch (not shown) is switched ON and then adisk-tray driving-force transmission mechanism (as set out below) isactivated, with the result that the disk tray 104 is inserted into thecasing 101 as shown in FIG. 21.

On the other hand, during ejection of the disk, the disk tray 104located in the casing 101 is ejected out of the casing 101 as shown inFIG. 20, by shifting an eject switch (not shown) to its operative statethrough application of pushing force to the eject button 140 in the diskloading state, or by shifting the disk-tray driving-force transmissionmechanism (as set out below) to its operative state in accordance withan eject signal generated from a host computer (not shown) in the diskloading state.

In the disk loading state shown in FIG. 38, in case of an emergencywherein electric power source cannot be switched ON owing to a serviceinterruption, failure in electric system or the like, the disk ejectingoperation is achieved as follows.

A wire-like emergency eject member 107 is inserted through the emergencyeject hole 102b of the front panel 102 in the direction of the arrow a(See FIG. 38).

At this time, as indicated in FIG. 25, the disk tray 104 is pushed outof the casing 101 by a constant distance L₁ in the direction of thearrow b by means of the emergency eject mechanism set out below.

Thereafter, the disk tray 104 is pulled out in the direction of thearrow b, by taking the front plate 106 out.

Also provided on the bottom surface of the disk placing plate 105 of thedisk tray 104 is a substantially J-shaped inside guide groove 108 and asubstantially J-shaped outside rack 109 being close to each other andspaced apart from each other by a predetermined distance r.

The rack 109 is constructed by a straight portion 109a extending in thetray loading direction (the direction of the arrow a) and in the trayeject direction (the direction of the arrow b), a first circular portion109b connected to the straight portion 109a at the straight-portion endlying in the tray eject direction, and a second circular portion 109cconnected to the first circular portion. The guide groove 108 isconstructed by a straight portion 108a extending in the tray loadingdirection and tray eject direction, a first circular portion 108bconnected to the straight portion 108a at the straight-portion end lyingin the tray eject direction, and a second circular portion (See FIG.28). 108c connected to the first circular portion.

The radius R₂ of curvature of the center line of the second circularportion 108c of the guide groove 108 is so dimensioned as to be equal tothe turning radius of a guide pin (as set out below) which pin isrotatable about the center indicated by O₂ in both directions indicatedby the arrows e and f. The radius R₁ (about the center O₁) of curvatureof the center line of the first circular portion 108b is set to besmaller than the radius R₂ of curvature of the second circular portion108c.

Reference numeral 30 denotes a synthetic-resin chassis mounted in thecasing 101. The chassis is comprised of a base plate 31 having anopening 31a substantially at its center and two opposing right and leftside wall plates 32 connecting to the base plate 31 (See FIGS. 25 and26).

Two mounting-pins 35 and 36, which extend upwardly and respectively haveflanged portions 35a and 36a, are screwed onto the rear section of thebase plate 31 of the chassis 30 so that these pins are arranged inparallel with each other by a predetermined interval in the lateraldirection of the chassis.

Additionally, the base plate 31 of the chassis 30 is formed with a guidegroove 31b provided near by the back of the emergency eject hole 102band extending in the tray-loading and tray-eject directions.

On the other hand, each side wall plate 32 of the chassis 30 isintegrally formed with a guide 33 which extends in the tray-loading andtray-eject directions and has a concave groove 33a engaging thepreviously-noted projected portion 105c (See FIG. 30).

A pulley holder 136 is provided among the upper ends of the side wallplates 32. The pulley holder is located above the internal spacenecessary to insert the disk tray 104 into the casing and has a throughopening 136a at the center of the holder (See FIG. 35).

Reference numeral 110 denotes a synthetic-resin holder driving leverwhich is provided at the right-hand side of the chassis 30 at thechassis end lying in the tray eject direction in such a manner as to berotatable in the directions indicated by the arrows c and d (See FIG.31).

The holder driving lever 110 is integrally formed with a support pin110a at one side end thereof and a stepped portion 111 at the other sideend thereof. The support pin is supported by means of bearings 30a andprojects in the directions indicated by the arrows a and b. The steppedportion is provided to support the front end of a unit holder 40. Thestepped portion has a pin mounting hole 111a being threadably engagedwith a mounting-pin 34 which pin has a flanged portion 34a and projectsin the same direction as the previously-noted mounting-pins 35 and 36 inthe upward holder position.

The holder driving lever 110 is integrally formed with a lever operatingpin 110b at its unsupported end, so that the lever operating pinprojects in the tray eject direction (the direction indicated by thearrow b).

Reference numerals 37 to 39 denote shock-absorbing rubbers serving asinsulators. The rubbers are respectively comprised of cylindricalportions 37a to 39a (represented by only the cylindrical portion 37a)opening in their vertical direction and a pair of upper and lowerexpanded portions 37b to 39b, each pair connecting to the correspondingone of the cylindrical portions 37a to 39a. The rubbers are disposed onthe outer peripheries of the respective mounting pins 34 to 36 andpress-fitted onto the chassis 30 by means of the flanged portions 34a to36a (See FIGS. 27 and 32 (A) and (B).

Reference numeral 40 denotes the unit holder having an opening 40apenetrating in its vertical direction and a cut-out 40b connected to theopening 40a. The unit holder is supported on the chassis 30 by means ofthe mounting pins 34 to 36. The unit holder is rotatably provided in thedirections indicated by the arrows c and d of FIGS. 35 and 36, by meansof the holder driving lever 110. The unit holder is integrally formedwith a holding plate 41 projecting from its right-hand end.

Formed nearby the center of the front end of the unit holder 40 and atboth sides of the rear end of the unit holder are cut-outs 42 to 44,facing the respective cylindrical portions 37a to 39a of theshock-absorbing rubbers 37 to 39 (See FIGS. 24 and 25).

Reference numerals 51 and 52 denote left and right guide shaftsextending in the front and rear directions (the directions indicated bythe arrows a and b) of the chassis and arranged in parallel with eachother. The guide shafts are fixedly connected to the bottom of the unitholder 40. Of these, the guide shaft 52 is mounted so that a portion ofthe guide shaft facing the above-noted opening 40a (See FIGS. 2, 4, and25).

Reference numeral 112 denotes a slider necessary for emergency ejectingoperation. The slider is comprised of a horizontal plate 112a and avertical plate 112b. For instance, these plate are integrally formed ofsynthetic resin to provide a substantially L-shaped slider. The slideris provided to be slidable in the guide groove 31b in its advancing andretreating directions (See FIGS 32 (A) and (B)).

The vertical plate 112a of slider 112 is formed with a concavity 113facing the tray ejecting side of the panel.

Reference numeral 114 denotes the disk-tray driving-force transmissionmechanism necessary for loading and ejecting operations of the tray. Thedisk-tray driving-force transmission mechanism is comprised of areduction gear mechanism 116 driven by a drive motor 115, asubstantially L-shaped gear base 117, and a boomerang-shaped cam lever118. The gear base is linked to the reduction gear mechanism 116 andsupported on the chassis 31 by means of the support shaft 31c to berotatable in the directions indicated by the arrows e and f. The gearbase has a sectorial gear 117a at its one end. The cam lever is linkedto the gear base 117 and supported on the chassis 31 by means of thesupport shaft 31d to be rotatable in the directions indicated by thearrows g and h. The cam lever has a sectorial gear 118a which is broughtinto meshed contact with the sectorial gear 117a (See FIG. 31).

The gear base of the disk-tray driving-force transmission mechanism 114is formed of synthetic resin and comprised of a cylindrical boss 117b, aguide pin 117c, and a support pin 117d disposed substantially midwaybetween the cylindrical boss 117 and the guide pin 117c. The cylindricalboss projects upwardly in the vicinity of the sectorial gear 117a.Additionally the support shaft 31c is inserted into the boss. The guidepin 117c is formed at the gear-base end facing apart from the othergear-base end formed with the boss 117b in such a manner to projectupwardly and fitted into the above-noted guide groove 108.

The reduction gear mechanism 116 of the disk-tray driving-forcetransmission mechanism 114 is comprised of a driving pulley 119, adriven pulley 122, a two-stepped intermediate gear 123, and a steppedoutput gear 124. The driving pulley is fixed onto the output shaft 115aof the drive motor 115. The driven pulley 122 is linked to the drivingpulley 119 through a transmission belt 120, and rotatably supported bythe support pin 117d, and has an input gear 121. The intermediate gearincludes a large-size gear 123a and a small-size gear 123b integrallyformed with each other. The large-size gear 123a is linked to the drivenpulley 122, is rotatably supported by the support pin 117d, and mesheswith the input gear 121. The output gear 124 includes a large-size gear124a which is linked to the intermediate gear 123 and rotatablysupported by the guide pin 117c and meshes with the small-sized gear123b, and also includes a small-size gear 124b which meshes with theabove-noted rack 109.

The input gear 121 of the reduction gear mechanism 116 consists of a sungear, while each of the output gear 124 and the intermediate gear 123consists of a planetary gear which is in circular motion about the inputgear 121.

The cam lever 118 of the disk-tray driving-force transmission mechanism114 is integrally formed with a substantially cylindrical boss 118bwhich boss is provided at the intermediate portion of the lever in sucha manner as to project upwards. Additionally the support shaft 31d isinserted into the boss.

The cam lever 118 is integrally formed with a cam 120 and a slideroperating-force receiving pin 120a. The cam extends from the boss 118bin the radial direction of the boss and has a substantially Z-shapedconcave groove 119 into which the lever operating pin 110b of the holderdriving lever 110 is fitted. The slider operating-force receiving pinprojects downwards at the lever end opposing the sectorial gear 118a,and is engageable with the tray-loading side surface of the verticalplate 112b of the above-noted slider 112 (See FIG. 31).

The concave groove 119 of the cam lever 118 is comprised of a high-levelportion 119a, a low-level portion 119b and a sloped portion 119cinterconnecting the high-level portion 119a and the low-level portion119b. Of these, the low-level portion 119b is dimensioned so that thelow-level portion has a comparatively wide size in the circumferentialdirection of the above-noted boss 118b.

Reference numeral 53 denotes an optical head carriage required fortransferring an optical pick-up device 54 in the tray loading direction(the direction indicated by the arrow a) and in the tray eject direction(the direction indicated by the arrow b). The optical head carriage isslidably supported on the guide shafts 51 and 52 through bearings 45 and46.

The optical head carriage 53 is integrally formed with raised blocks 53band 53c at its right-hand side so as to support the above-noted bearing46. The raised blocks are juxtaposed to each other through a connectionwall 53a and extend in the tray eject direction and the tray loadingdirection (See FIGS 1-4 and 24-25).

Reference numeral 124 denotes a rack support pin projecting upwards. Therack support pin is press-fitted to a portion which is located at theright-hand side of the optical head carriage in a manner to be slightlyapart from the raised blocks 53b and 53c towards the left-hand sideguide shaft 51 of the guide shafts 51 and 52.

The upper end of the rack support pin 124 is formed with an annulargroove 124a into which a snap ring 125 is detachably fitted so as tomount the rack (See FIG. 4).

Reference numeral 55 denotes the rack necessary to transmit a drivingforce produced by the motor. The rack is comprised of a rack plate 55a,a rack mounting plate 55b and a L-shaped connection plate 55c. The rackplate 55a includes at its front surface a rack function portion 55a₁which extends in the tray loading direction (the direction indicated bythe arrow a) and in the tray eject direction (the direction indicated bythe arrow b). The rack mounting plate is provided at the back of the endof the rack plate 55a which end corresponds to one end in the tray ejectdirection, so that the rack mounting plate opposes the upward end of theoptical head carriage 53. The connection plate 55c is disposed betweenthe rack mounting plate 55b and the rack plate 55a, and formed of ahorizontal piece 55c₁ and a vertical piece 55c₂. The rack is rotatablysupported by the rack support pin 124 in directions indicated by thearrows a₁ and a₂, in a manner so as to cover a portion of the guideshaft 52 (See FIG. 1).

The rack plate 55a is formed with a cut-out 126 at the back of therack-plate end in the tray eject direction and a projected portion 127opposing the above-noted connection wall 53a. The cut-out faces aportion of the respective raised blocks 53b and 53c, while the projectedportion faces an intermediate section of the cut-out 126. The rack plateis integrally formed with a projected portion 128 at the back of therack-plate end in the tray-loading direction. The projected portionextends in the direction perpendicular to the above-noted guide shafts51 and 52 and has a top end surface corresponding to a segment of anouter periphery of a cylinder (See FIGS. 16(A), (B) and (C) and FIG. 1).

The rack mounting plate 55b is integrally formed with a cylindrical boss129 at its right-hand side upper end surface, into which boss the racksupport pin 124 is inserted.

The connection plate 55c is integrally formed with a L-shaped piece 130at its end in the tray loading direction. The L-shaped piece iscomprised of a horizontal portion 130a projecting in the directionopposite to the rack plate 55a and a vertical portion 130b opposing thereverse of the rack plate 55a via the guide shaft 52.

The vertical portion 130b of the L-shaped piece 130 is integrally formedwith an additional projected portion being provided at a positionslightly offsetting in the tray eject direction (the direction indicatedby the arrow b) from a portion opposing the projected portion 128. Theadditional projected portion 131 extends in the direction perpendicularto the guide shafts 51 and 52 and has a top end surface corresponding toa segment of an outer periphery of a cylinder.

Reference numeral 56 denotes a spindle motor having a motor shaft 56awhich shaft penetrates the cut-out 40b. The spindle motor is fixed onthe forward end of the unit holder 40 and close to the left-hand side ofthe unit holder in such a manner as to project the reverse of the unitholder (See FIGS. 4, 5, 23, 25).

Reference numeral 57 denotes a disk table on which an optical disk isplaced. The disk table is fixedly connected onto the upper end of themotor shaft 56a.

Reference numeral 132 denotes a chucking pulley which is capable ofpressing the non-recording zone of the optical disk D against the disktable 57. The chucking pulley is loosely fitted into a through hole 136ain such a manner as to be movable in the vertical direction and in thehorizontal direction.

The outer periphery of the chucking pulley 132 is integrally formed witha flanged portion 132a opposing a flanged portion 136b of a pulleyholder 136. A through hole 132b is formed in the central portion of thechucking pulley 132 in a manner so as to open in the vertical direction.(See FIG. 36.)

Attached to the chucking pulley 132 is a metallic plate 133 made ofaluminum for example, which plate is provided to close the opening ofthe through hole 132b.

Reference numeral 58 denotes a carriage driving motor having a motorgear 58a exposing towards the under of the holder. The carriage drivingmotor is fixedly connected to the unit holder 40 at the rearward end ofthe holder and at the right-hand side of the holder (See FIG. 3).

Reference numeral 134 denotes a ring provided for detecting a rotationalangle of the motor gear 58a.

Reference numeral 59 denotes a drive-gear mechanism having a drive gear59a which is rotatable about a support shaft 41c through rotation of themotor gear 58a. The drive-gear mechanism is disposed at the reverse sideof the holding plate 41.

Reference numeral 60 denotes an intermediate transmission mechanismprovided for transmitting a driving force produced by the driving motor58 to the head carriage 53 through the rack 55. The intermediatetransmission mechanism is linked to the drive gear mechanism 59 and hasa driving-side transmission mechanism 61 having a transmission gear Aand a driven-side transmission mechanism 62 having a transmission gearB. Both transmission gears rotate through rotation of the driving gear59a. The intermediate transmission mechanism is provided at the outsideof the right-hand side guide shaft 52 of the guide shafts 51 and 52.

The transmission gear of the driven-side transmission mechanism 62meshes with the rack 55 and has a first driven-side transmission gear62a and a second driven-side transmission gear 62b, the respective gearsbeing rotatable independently of each other. This transmission gear B isrotatably supported at the back end surface of the holding plate 41 byway of the support shaft 41a.

The first driven-side transmission gear 62a of the transmission gear Bis formed with a through hole 62A₁ penetrating its upper and lowersurfaces and a projected portion 62A₂ projecting downwards (See FIG. 4).

The second driven-side transmission gear 62b of the transmission gear Bis formed with a through hole 62B₁ penetrating its upper and lowersurfaces and a frame-like portion 62B₂ loosely fitted into the projectedportion 62A₂.

In the second driven-side transmission gear 62b and the firstdriven-side transmission gear 62a, when the first and second driven-sidetransmission gears 62a and 62b are assembled to each other such that theprojected portion 62A₂ is loosely fitted into the frame-like portion62B₂, as shown in FIG. 5, the respective transmission gears are designedto be rotatable in their circumferential direction by a maximum of 2degree in a state wherein the other peripheral gears are not meshed withthem. Finally, in case that the peripheral gears are assembled withthem, the respective transmission gears are designed to be rotatable bya maximum of 0.4 degree.

The through hole 62B₁ and the through hole 62A₁ are designed to bethrough holes required for phase matching of gears during assembly. Withthe driven-side transmission mechanism 62 assembled, these through holesare located on the same plane and at the same circumference of a circle.

The transmission gear of the driving-side transmission mechanism 61 iscomprised of a first driving-side transmission gear 61a which mesheswith the drive gear 59a. The transmission gear of the driving-sidetransmission mechanism 61 is further comprised of a second driving-sidetransmission gear 61b which is linked to the first driving-sidetransmission gear 61a through a first backlash eliminating mechanism setout below and meshes with the drive gear 59a. The transmission gear ofthe driving-side transmission mechanism 61 is futher comprised of athird driving-side transmission gear 61c with a lever which gear isrotatably attached to the second driving-side transmission gear 61b andlinked to the first driving-side transmission gear 61a through a secondbacklash eliminating mechanism set out below and which meshes with thefirst driven-side transmission gear 62a. The transmission gear A isrotatably supported at the back end surface of the holding plate 41 byway of the support shaft 41a.

The driving-side transmission gear 61a of the transmission gear Aincludes a circular concavity 63 at its bottom end surface and alsoincludes a center hole 64 opening from the inside of the circularconcavity 63 to the outside. Integrally formed at the outer periphery ofthe upper opening of the center hole 64, is a small-size gear 65extending upwardly and meshing with the above-noted second driven-sidetransmission gear 62b (See FIG. 1).

Additionally, the first driving-side transmission gear 61a is formedwith two through windows 66 and 67 and two through holes 68 and 69,respectively opening from the inside of the circular concavity 63 to theoutside, in the same manner as the above-noted center hole 64.

These through windows 66 and 67 include a substantially boomerang-shapedthrough window and a substantially T-shaped through window,respectively. The peripheral edge of the reverse side of theboomerang-shaped through window is integrally formed with a springhanger 70 consisting of a vertical portion 70a projecting downwards anda horizontal portion 70b connected to the vertical portion 70a in such amanner as to partly close a portion of the through window. Theperipheral edge of the reverse side of the T-shaped through window isintegrally formed with a spring hanger 71 consisting of a verticalportion 71a projecting downwards and a horizontal portion 71b connectedto the vertical portion 71a in such a manner as to partly close aportion of the through window.

On the other hand, the through holes 68 and 69 are an essentiallycircular through hole and an essentially rectangular through hole,respectively. The peripheral edge of the reverse side of the throughhole 69 is integrally formed with an engaging portion 72 consisting of avertical portion 72a extending downwards and a horizontal portion 72bconnected to the vertical portion 72a (See FIGS. 1 and 2).

Hereinbelow explained is the second driving-side transmission gear 61b.The second driving-side transmission gear 61b is integrally formed witha circular concavity 73 opening towards the outside of the gear, throughholes 74a, 74b opening from the inside of the circular concavity 73 tothe outside and two substantially sectorial connection portions 75 and76 which extend within the opening of center hole 74 and arecircumferentially spaced apart from each other at regular intervals.

Also, the second driving-side transmission gear 61b is integrally formedwith a cylindrical boss 77 among the connection portions 75 and 76 sothat the boss penetrates both the above-noted center holes 64 and 74.The lower end of the boss, circumferentially offset from the respectiveconnection portions 75 and 76 by 90 degrees, is formed with twocut-outs, so as to provide stepped portions 78 and 79.

Furthermore, the second driving-side transmission gear 61b is formedwith two through windows 80 and 81 and two through holes 82 and 83,respectively opening from the inside of the circular concavity 73 to theoutside in the same manner as the above-noted center hole 74.

The through windows 80 and 81 include a substantially boomerang-shapedwindow into which a portion of the above-noted spring hanger 70 isprojected and a substantially rectangular window into which a portion ofthe above-noted spring hanger 71 is projected, respectively. Theperipheral edge outside of these through holes 80 and 81 are integrallyformed with a spring hanger extending upwards in such a manner as topartly close a portion of the through window.

On the other hand, the through holes 82 and 83 include an essentiallycircular through hole and a substantially trapezoidal through hole,respectively. The comer of the peripheral edge of the through hole 83 isintegrally formed with an engaging portion 85 extendingcircumferentially and corresponding to the horizontal portion 72b of theabove-noted engaging portion 72.

The back surface of the third driving-side transmission gear 61c isintegrally formed with a downwardly projecting cylindrical body 86 towhich the above-noted boss is fitted (See FIGS. 1-3). The lower end ofthe cylindrical body 86 is integrally formed with two resilient portions87 and 88 having respective nailed portions, 87a and 88a. Nailed portion87a engages with the above-noted stepped portion 78 and loosely fittedinto the through hole 74a of the second driving-side transmission gear61b. The other nailed portion 88a engages with the above-noted steppedportion 79 and is loosely fitted into the through hole 74b of the seconddriving-side transmission gear.

The cylindrical body 86 of the third driving-side transmission gear 61cis formed with a downwardly opening cut-outs 89 and 90 circumferentiallyarranged at regular intervals so that both cut-outs are adjacent to boththe resilient portions 87 and 88.

Additionally, a lever 91 is attached to the third driving-sidetransmission gear 61c. The lever consists of an annular base portion 91aopposing the above-noted center hole 64 and the peripheral edge of theunder side of the through holes 74a and 74b, and two arm portions 91band 91c, both connected to the outer peripheral edge of the base portion91a and respectively opposing the bottom surfaces of the circularconcavities 63 and 73.

The base portion 91a of the lever 91 is formed with a through hole 92Aopening from the inside of the lever to the outside and insertedtherethrough by both the resilient portions 87 and 88. The base portion91a of the lever 91 is aslo formed with projected portions 92 projectedinto the through hole 92A, exposed within the opening plane of thecenter hole 64 and fitted to the cut-outs 89 and 90.

Of these arm portions 91b and 91c of the lever 91, one arm portion 91bis integrally formed with a spring hanger 93 corresponding to the springhanger 70 and a stopper 94 projected into the through window 66. Theother arm portion 91c is formed with a through hole 95 being cooperativewith the through hole 68 of the first driving-side transmission gear 61aand the through hole 82 of the second driving-side transmission gear 61bfor the purpose of phase matching of toothed portions.

The through hole 95 and the through holes 68 and 82 (all having a boreof 1.5 mm) are designed to be through holes utilized for phase matchingamong toothed portions during assembly of the mechanisms. Under acondition of completion of assembling the driving-side transmission gear61, these through holes are all located at the identical area on theircircumference of the circle.

Reference numeral 135 denotes a thrust receiving plate having anengaging portion 135a engaging the guide shaft 52. The thrust receivingplate is fixedly secured to the holding plate 41 by means of screws 136.The thrust receiving plate is provided to prevent the drive gear 59a,the first, second and third driving-side transmission gears 61a to 61c,the lever 91, the first and second driven-side transmission gears 62aand 62b from falling out (See FIGS. 1-5).

The thrust receiving plate 135 is formed with a positioning hole 135b towhich the top end of the support shaft 41b is fitted, a screw-threadedholes 135c and 135d into each of which a screw is threadably engaged.

Hereinbelow explained is a meshing condition between the respectivegears.

Reference numeral 96 denotes a first coil spring serving as a firstbacklash eliminating means. The first coil spring is disposed betweenboth the spring hangers 71 and 84 in its permanently compressed state.Thus, as shown in FIG. 1, each of the spring hangers 71 and 84 isspring-loaded by way of reaction force of the first coil spring 96.

That is to say, as shown in FIG. 21, by the spring-bias of the firstcoil spring 96, a biasing force is applied to the first driving-sidetransmission gear 61a so that the first driving-side transmission gearrotates in the direction indicated by the arrow R₁, while a biasingforce is applied to the second driving-side transmission gear 61b sothat the second driving-side transmission gear rotates in the directionindicated by the arrow R₂.

For the reasons set forth above, the ridge of the drive gear 59a is keptin circumferentially meshed engagement with the ridge of the firstdriving-side transmission gear 61a at one side thereof and with theridge of the second driving-side transmission gear 61b at the other sidethereof.

The spring-bias of the first coil spring 96 is transmitted to both thefirst driving-side transmission gear 61a and the second driving-sidetransmission gear 61b as shown in FIG. 22(a), with the result that thefirst driving-side transmission gear 61a rotates clockwise to mesh withthe drive gear 59a as shown in FIG. 2, while the second driving-sidetransmission gear 61b rotates counterclockwise to mesh with the drivegear 59a. In this manner, the respective gears are kept in meshedengagement with each other.

On the other hand, reference numeral 97 denotes a second coil springserving as a second backlash eliminating means. The second coil springis disposed between the spring hangers 70 and 93 in its permanentlycompressed state. As shown in FIG. 1, each of the spring hangers isspring-loaded by way of reaction force of the second coil spring 97.

Referring now to FIG. 1, by the spring-bias of the second coil spring97, a biasing force is applied to the lever 91 having one spring hanger93 so that the lever rotates in the direction indicated by the arrow T.Thus, a biasing force is applied to the third driving-side transmissiongear 61c fixedly fitted to the lever 91, so that the third driving-sidetransmission gear rotates in the direction indicated by the arrow T.

As a result of the biasing force produced by the second coil spring 97and acting on the third driving-side transmission gear 61c in thedirection indicated by the arrow T, the biasing force is applied to thefirst driven-side transmission gear 62a meshing with the thirddriving-side transmission gear 61c so that the driven-side transmissiongear rotates in the direction indicated by the arrow S₁.

In the same manner, as seen in FIG. 1, the spring-bias of the secondcoil spring 97 acts to rotate the first driving-side transmission gear61a with the other spring hanger 70 in the direction indicated by thearrow R₁, and thus the biasing force is applied to the small-size gear65 which is rotatable together with the first driving-side transmissiongear 61a, with the result that the small-size gear rotates in thedirection indicated by the arrow R₁.

As a result of the biasing force produced by the second coil spring 97and acting on the small-size gear 65 in the direction indicated by thearrow R₁, the biasing force is applied to the second driven-sidetransmission gear 62b so that the second driven-side transmission gearrotates in the direction indicated by the arrow S₂.

That is to say, as shown in FIG. 20, by the spring-bias of the secondcoil spring 97, a biasing force is applied to the first driven-sidetransmission gear 62a so that the first driven-side transmission gearrotates in the direction indicated by the arrow S₁, while a biasingforce is applied to the second driven-side transmission gear 62b so thatthe second driven-side transmission gear rotates in the directionindicated by the arrow S₂.

For the reasons set out above, the ridge of the rack 55 is kept incircumferentially meshed engagement with the ridge of the firstdriven-side transmission gear 62a at one side thereof and with the ridgeof the second driven-side transmission gear 62b at the other sidethereof.

Additionally, owing to the spring-bias of the second coil spring 97, asseen in FIG. 22(a), a biasing force is applied, on the one side, to thefirst driven-side transmission gear 62a through the lever 91 and thesecond driving-side transmission gear 61c, and a biasing force isapplied, on the other hand, to the second driven-side transmission gear62b through the first driving-side transmission gear 61a.

As a consequence, the first driven-side transmission gear 62a rotatesclockwise to mesh with the rack 55 as seen in FIG. 2, while seconddriven-side transmission gear 62b rotates counterclockwise to mesh withthe drive gear 59a as seen in FIG. 2. In this manner, the respectivegears are kept in meshed engagement with each other.

As appreciated from the above, the first and second coil springs 96 and97 are cooperative to each other so that the first and seconddriving-side transmission gears 61a and 61b are kept in meshedengagement with the drive gear 59a, while rotating in the two opposingcircumferential directions with respect to the drive gear. Consequently,backlash free engagement is permanently maintained after assembling thegears.

Hereinbelow explained in detail is a process of transmission of drivingforce from the driving motor 58 to the rack 55.

First of all, as seen in FIG. 1, explained is such a case where thedriving motor 58 rotates in its positive direction.

Rotation of the driving motor 58 to the positive direction results inrotation of the motor gear 58a in the direction indicated by the arrowU₁, and thereby results in rotation of the drive gear 59a in thedirection indicated by the arrow V₁.

Referring now to FIG. 21, since the driving force acting on the drivewheel 59a in the direction indicated by the arrow V₁ transmitted tosecond driving-side transmission gear 61b which gear meshes with thedrive gear 59a by rotating the second driving-side transmission gear inthe direction indicated by the arrow R₂ by way of the biasing forceconstantly acting in the direction of the arrow R₂, the seconddriving-side transmission gear 61b rotates in the direction indicated bythe arrow r₂ illustrated in FIG. 1.

When the second driving-side transmission gear 61b rotates in thedirection indicated by the arrow r₂, the rotational force acting in thedirection of the arrow r₂ is transmitted to the first driving-sidetransmission gear 61a against the spring bias of the first coil spring96, with the result that the first driving-side transmission gear 61arotates in the same rotational direction as the second driving-sidetransmission gear 61b, i.e., in the direction indicated by the arrow R₁.

The first coil spring 96 is so designed to have a compression forcestrong enough to counter against the driving force produced by rotationof the second driving-side transmission gear 61b. In the event that thedriving force is transmitted from the second driving-side transmissiongear 61b through the first coil spring 96 to the first driving-sidetransmission gear 61a, the first coil spring 96 cannot be compressedmore than the pre-compressed state of the first coil spring afterassembly.

In accordance with rotation of the first driving-side transmission gear61a to which the driving force is applied in the direction of R₁, thesmall-size gear 65 rotates together with the first driving-sidetransmission gear 61a in the same direction as the first driving-sidetransmission gear, and thus the driving force is transmitted to thesecond driven-side transmission gear 62b meshing with the small-sizegear 65, thereby resulting in rotation of the second driven-sidetransmission gear 62b in the direction indicated by the arrow S₂.

Referring to FIG. 20, since the biasing force acting in the direction ofthe arrow S₂ is applied constantly to the second driven-sidetransmission gear 62b. as set forth above, the second driven-sidetransmission gear meshes with the rack 55, while rotating in thedirection of the arrow S₂.

That is to say, when the driving force acting in the direction of thearrow S₂ is applied to the second driven-side transmission gear 62b bythe driving force produced by rotation of the driving motor 58 in itspositive direction, this driving force is transmitted directly to therack 55.

Thus, in case that the driving force produced by rotation of the drivingmotor 58 to the positive direction is transmitted to the rack 55, thereis no occurrence of backlash owing to an error of meshed engagement oftoothed portions of the gears.

Returning to FIG. 1, explained is such a case where the driving motor 58rotates in its negative direction.

Rotation of the driving motor 58 to the negative direction results inrotation of the motor gear 58a in the direction indicated by the arrowU₂, and thereby results in rotation of the drive gear 59a in thedirection indicated by the arrow V₂.

Returning to FIG. 21, since the driving force acting on the drive wheel59a in the direction indicated by the arrow V₂ transmitted to firstdriving-side transmission gear 61a which gear meshes with the drive gear59a by rotating the first driving-side transmission gear in thedirection indicated by the arrow R₁ by way of the biasing forceconstantly acting in the direction of the arrow R₁, the firstdriving-side transmission gear 61a rotates in the direction indicated bythe arrow r₁ illustrated in FIG. 1.

When the first driving-side transmission gear 61a rotates in thedirection indicated by the arrow r₁, the rotational force acting in thedirection of the arrow r₁ is transmitted to the lever 91 against thespring bias of the second coil spring 97.

In the same manner as the first coil spring 96, the second coil spring97 is also designed to have a compression force strong enough to counteragainst the driving force produced by rotation of the first driving-sidetransmission gear 61a. In the event that the driving force istransmitted from the first driving-side transmission gear 61a throughthe second coil spring 97 to the lever 91, the second coil spring 97cannot be compressed more than the pre-compressed state of the secondcoil spring after assembly.

The driving force applied to the lever 91 results in rotation of thelever 91 in the direction indicated by the arrow T as seen in FIG. 1.Then, the third driving-side transmission gear 61c firmly fitted to thelever 91 rotates in the direction of the arrow T.

Via the third driving-side transmission gear, the driving force istransmitted to the first driven-side transmission gear 62a meshing withthe third driving-side transmission gear 61c, with the result that thefirst driven-side transmission gear 62a rotates in the directionindicated by the arrow S₁.

Referring now to FIG. 20, as set out above, since the biasing forceacting in the direction of the arrow S₂ is constantly applied to thefirst driven-side transmission gear 62a, the first driven-sidetransmission gear meshes with the rack 55, while rotating in thedirection of the arrow S₁.

That is to say, when the driving force acting in the direction of thearrow S₁ is applied to the first driven-side transmission gear 62a bythe driving force produced by rotation of the driving motor 58 in itsnegative direction, this driving force acting in the direction of thearrow S₁, is transmitted directly to the rack 55.

Therefore, in case that the driving force produced by rotation of thedriving motor 58 to the negative direction is transmitted to the rack55, there is no occurrence of backlash owing to an error of meshedengagement of toothed portions of the gears, in the same manner as thetransmission of driving force to the rack 55 with the driving motorrotating to its positive direction.

That is to say, irrespective of positive and negative rotations of thedriving motor 58, the first and second driving-side transmission gears61a and 61b both mesh with the drive gear 59a with the aid of thespring-bias of the first coil spring 96, while the first and seconddriven-side transmission gears 62a and 62b mesh with the rack 55 withthe aid of the spring-bias of the second coil spring 97. Under theabove-mentioned particular meshing conditions, there is no occurrence ofbacklash.

Hereinbelow explained are the transmission mechanism of the above-noteddriving force and the principle of absorption of backlash, in accordancewith FIG. 22(b), diagrammatically indicating the flow of driving force.

First, in case that the driving motor 58 rotates in the positivedirection, the driving force produced by rotation of the motor istransmitted to the drive gear 61b, and further to first driving-sidetransmission gear 61a against the spring-bias of the first coil spring96.

That is to say, in case that the driving motor 58 rotates in thepositive direction, it will be appreciated that a first driving-forcetransmission path set forth above is formed.

Alternatively, in case that the driving motor 58 rotates in the negativedirection, the driving force produced by rotation of the motor istransmitted from the drive gear 59a to the first driving-sidetransmission gear 61a, and further to both the lever 91 and the thirddriving-side transmission gear 61c against the spring-bias of the secondcoil spring 97.

After being transmitted to the first driving-side transmission gear 61a,the driving force is transmitted finally to the rack 55.

That is to say, in case that the driving motor 58 rotates in thenegative direction, it will be appreciated that a second driving-forcetransmission path set forth above is formed.

In the above-noted first transmission path, the meshed engagementbetween the drive gear 59a and the second driving-side transmission gear61b, the meshed engagement between the first driving-side transmissiongear 61a and the second driven-side transmission gear 62b, and themeshed engagement between the second driven-side transmission gear 62band the rack 55 will be hereinafter referred to as "engagement A", (i.e.MESHA)"engagement B", and "engagement C", respectively.

In the above-noted second transmission path, the meshed engagementbetween the drive gear 59a and the first driving-side transmission gear61a, the meshed engagement between the third driving-side transmissiongear 61c and the first driven-side transmission gear 62a, and the meshedengagement between the first driven-side transmission gear 62a and therack 55 will be hereinafter referred to as "engagement D", "engagementE", and "engagement F", respectively.

The first transmission path formed in case that the driving force istransmitted by rotation of the driving motor 58 to the positivedirection, is constructed by only the engagements A, B and C in threedifferent places. On the other hand, the second transmission path formedin case that the driving force is transmitted by rotation of the drivingmotor 58 to the negative direction, is constructed by only the threeengagements D, E and F, each being different from any one of theengagements A, B and C.

That is to say, with regard to the respective meshed engagements A, B,C, D, E and F, since the toothed portions of the gears can be meshed totransmit the driving force only to one direction of the positive andnegative directions, there is no occurrence of backlash when switchingthe driving direction among the positive direction and the negativedirection.

Therefore, in the present embodiment, since backlashes a to c betweenthe drive gear 59a and the rack 55 are eliminated by means of the firstcompression coil spring 96 and the second coil spring 97 both includedin the driving-side transmission mechanism 61, only phase matching atthe driving-side transmission mechanism 61 is required in order toachieve phase matching at the toothed portions of the gears in thetransmission mechanism with the backlash eliminating mechanism, wherebyassembling task can be simplified and assembly time can be decreased.

Additionally, in the present embodiment, since the compression coilspring is not included in the rack 55 as compared with the prior art,the rack 55 is so designed to be small-sized in its width direction.Thus, when transferring the optical pick-up device 54, the moment of aforce about the meshed engagement point with regard to the optical headcarriage 53 can be set at the minimum, thereby reducing transfer loss inthe head.

Furthermore, in the present embodiment, since the dimension of the rack55 in its width direction is set to be small, the entire dimension ofthe unit can be small-sized in its width direction.

In addition to the above, in the embodiment, the provision of thethrough holes 68, 82 and 95, respectively formed in the firstdriving-side transmission gear 61a, the second driving-side transmissiongear 61b, and the lever 91, is advantageous to be able to achieve phasematching at the toothed portions of the transmission gears 61a to 61cincluded in the driving-side transmission gear mechanism 61 by matchingall openings of the through holes 68, 82 and 95 by way of insertion of apin into them. This ensures more easy assembling work in the mechanisms.

In the embodiment, since the engaging portions 72 and 85 are providedrespectively in the first driving-side transmission gear 61a and thesecond driving-side transmission gear 61b, these transmission gears canbe integrally linked to each other by way of both the engaging portions72 and 85. In view of this, it will be appreciated that the driving-sidetransmission gear mechanism 61 can be easily assembled

Moreover, when the optical head carriage 53 moves in the tray loadingdirection (the direction indicated by the arrow a) along both the guideshafts 51 and 52, the rack 55 receives a side pressure F through theintermediate transmission mechanism 60 by means of the first and secondcoil springs 96 and 97 during transferring from the innermost peripheraldisk-position shown in FIG. 23 to the outermost peripheral disk-positionshown in FIG. 24. However, since the rack is rotatable about the racksupport pin 124, the rack can receive the side pressure such that theside pressure is dispersed at the setting positions of the bearings 45and 46 and at the setting position of the projected portion 127. As aresult, a smooth motion of the optical head carriage 53 is assured.

In this case, the moment of a force about the rack support pin 124,which moment acts on the rack 55 owing to the side pressure F, theshortest distance (the arm of moment) from the point of act (the meshingpoint between the rack 55 and either one of the first and seconddriven-side transmission gears 62a and 62b) of the side pressure F tothe fulcrum (the rack support pin 124) becomes maximum at the innermostperipheral disk-transfer position of the rack 55 as shown in FIG. 23 andbecomes minimum at the outermost peripheral disk-transfer position asshown in FIG. 24.

Hereinbelow explained is a process according to another aspect of theinvention for assembling a transmission mechanism which mechanism drivesthe head carriage, utilizing FIGS. 1, 2, 4 and 5.

First, the first driven-side transmission gear 62a and the seconddriven-side transmission gear 62b are inserted in turns into the supportshaft 41a, so that the driven-side transmission mechanism 62 is broughtinto meshed engagement with the rack 55.

At this time, the first driven-side transmission gear 62a and the seconddriven-side transmission gear 62b are assembled to each other in such amanner that the through hole 62A₁ of the first driven-side transmissiongear 62a and the through hole 62B₁ of the second driven-sidetransmission gear 62b are matched to each other, and additionally theprojected portion 62A₂ of the first driven-side transmission gear 62a isfitted to the frame-like portion 62B₂ of the second driven-sidetransmission gear 62b.

Subsequently, the drive gear 59a is inserted into the support shaft 41cso that the drive-gear mechanism 59 is brought into meshed engagementwith the motor gear 58a of the driving motor 58.

Thereafter, the driving-side transmission mechanism 61 is interleavedbetween the drive-gear mechanism 59 and the driven-side transmissionmechanism 62.

At this juncture, after the cylindrical body 86 of the thirddriving-side transmission gear 61c has been inserted into the centerhole 64 of the first driving-side transmission gear 61a, the respectiveresilient portions 87 and 88 of the inserted end are inserted into thethrough hole 92A of the lever 91. After the projected portion 92b of thelever 91 has been fitted into the cut-outs 89 and 90, the boss 77 of thesecond driving-side transmission gear 61b is placed into the cylindricalbody 86, and the nailed portions 87a and 88a are engaged with thestepped portions 78 and 79 by loosely fitting the resilient portions 87and 88 into the through holes 74a and 74b.

At this time, the first driving-side transmission gear 61a, the seconddriving-side transmission gear 61b, the third driving-side transmissiongear 61c and the lever 91 are assembled with each other in such a mannerthat the through hole 68 of the first driving-side transmission gear61a, the through hole 82 of the second driving-side transmission gear61b and the through hole 95 of the lever 95 are matched to each other,and additionally the engaging portion 72 (the horizontal portion 72b) ofthe first driving-side transmission gear 61a is fitted to the engagingportion 86 of the second driving-side transmission gear 61b.

Thereafter, the support shaft 41b is fitted into the positioning hole135b of the thrust receiving plate 135, and then the engaging portion135a is engaged with the guide shaft 52. The thrust receiving plate 135is fixed on the holding plate 41 by way of screws 136.

Thereafter, the first compression coil spring 96 is hung between thespring hanger 71 (the horizontal portion 71b) of the first driving-sidetransmission gear 61a and the spring hanger 84 of the seconddriving-side transmission gear 61b, such that the spring is disposedwithin the through window 67 of the transmission gear 61a and within thethrough window 81 of the transmission gear 61b, while the secondcompression coil spring 97 is hung between the spring hanger 70 (thehorizontal portion 70b) of the first driving-side transmission gear 61aand the spring hanger 93 of the lever 91, such that the spring isdisposed within the through window 66 of the transmission gear 61a andwithin the through window 80 of the transmission gear 61b.

In this manner, the transmission mechanism driving the head carriage canbe assembled.

Hereinbelow explained are a disk chucking operation, a disk reproductionoperation, a disk chucking release operation, a disk-tray loadingoperation, an eject operation, and an emergency eject operation, in theoptical disk unit according to the present embodiment.

Disk Chucking Operation

As shown in FIG. 35, under a condition in which the unit holder 40descends to its descended position in the direction indicated by thearrow c owing to dead load, the disk D is horizontally inserted into thecasing 101 in the direction of the arrow a by way of the disk tray 104,so that the disk is positioned between the disk table 57 and thechucking pulley 132.

After completion of loading the disk tray 104, the holder driving lever110 rotates in the direction of the arrow d, with the result that theunit holder 40 also rotates in the direction of the arrow d until theholder reaches its ascended position and then the holder is kept in ahorizontal position, as seen in FIG. 36.

At this time, the disk table 57 put on the unit holder 40 is insertedinto the concavity 105a of the disk tray 104 through the opening 105b inthe direction of the arrow d, and the disk table is thus fitted into thecenter hole D₁ of the optical disk D, and additionally, the optical diskD is lifted up within the concavity 105a of the disk tray 104.

The disk table 57 lifts up the chucking pulley 132 together with theoptical disk D and simultaneously operates to attract a metal plate 133of the chucking pulley 132 downwards by means of a chucking magnet (notshown) placed on the disk table 57.

Additionally, the chucking pulley 132 magnetically absorbs the perimeterof the center hole D₁ of the optical disk D onto the disk table 57.

Disk Reproduction Operation

After the optical disk D has been chucked, the optical disk D rotatestogether with the disk table 57 by means of the spindle motor 56, andthen information stored in the optical disk D is reproduced by theoptical pick-up device 54 while traveling the optical head carriage 53in the direction indicated by the arrows a or b by means of the drivingmotor 58

Disk Chucking Release Operation

After reproduction of the optical disk D, when the holder driving lever110 rotates in the direction of the arrow c, and then the unit holder 40descends again in the direction of the arrow c with the aid of dead loadas seen in FIG. 35, the disk table 57 is removed out of the center holeD₁ of the optical disk D and further descends to the downward positionof the disk tray 104 in the direction of the arrow c.

Then, the optical disk D is relocated within the concavity 105a of thedisk tray 104. Additionally, the chucking pulley 132 is held above theoptical disk D by abutting the flanged portion 132a with the peripheraledge of the through hole 136a of the pulley holder 136.

Thereafter, the optical disk D lying in the casing 101 is ejected out ofthe casing 101 in the direction of the arrow b together with the disktray 104.

Disk-tray Loading Operation

Under a condition of completion of the ejecting operation, wherein thedisk tray 104 is transferred in the direction of the arrow b as shown inFIG. 37, the small-size gear 124b and the guide pin 117c are bothpositioned at the end of the straight portion 109a of the rack 109 ofthe disk tray 104 and the straight portion 108a of the guide groove 108of the disk tray, which end corresponds to the end in the tray loadingdirection a, as indicated by the two-dotted line of FIG. 29.

In the above-noted positioned state, when the loading switch (not shown)is switched ON, the driving motor 115 rotates in its positive directionas seen in FIGS. 25 to 27, and as a result the small-size gear 124b ofthe output gear 124 rotates in the direction indicated by the arrow m atthe position indicated by the two-dotted line of FIG. 29.

In this case, the guide pin 117c engages with the straight portion 108aof the guide groove 108. As a result of the engagement, free rotationabout the support shaft 31c of the gear base 117 in both directionsindicated by the arrows e and f, is restricted.

Thus, the small-size gear 124b rotates in the direction of the arrow mat the restricted position, with the result that the small-size gear124b drives the rack in the direction of the arrow a along the straightportion 109a of the rack 109.

Then, the disk tray 104 is inserted into the casing 101 together withthe rack 109 in the direction of the arrow a, while the small-size gear124a and the guide pin 117c are cooperative to relatively move thestraight portion 108a of the guide groove 108 in the direction of thearrow b.

In accordance with advancement of loading of the disk tray 104 in thedirection of the arrow a, the small-size gear 124b and the guide pin117c reach the intersecting point P₁ between the center line of thestraight portion 108a of the guide groove 108 and the center line of thefirst circular portion 108b, as shown in the solid line of FIGS. 25 and29.

Thereafter, by way of further positive rotation of the small-size gear124b in the direction of the arrow m, the small-size gear 124b rolls inthe direction of the arrow f along the first circular portion 109b ofthe rack 109. The gear base 117 rotates with the support shaft 31c asthe center O₂ in the direction of the arrow f. The guide pin 117cadvances along the first circular portion 108b of the guide groove 108in the direction of the arrow f.

At this time, since the radius R₁ of curvature of the center line of thefirst circular portion 108b of the guide groove 108 is so designed to besmaller than the turning radius of the guide pin 117c with the supportshaft 31c as the center O₂, a traveling velocity of the disk tray 104 tothe direction of the arrow a decreases, as the guide pin 117c advancesalong the first circular portion 108b in the direction of the arrow f.

That is to say, as seen in FIG. 29, set at approximately 10.5 mm forexample, is the distance L₁ between two intersecting points P₁ and P₂.One intersecting point P₂ is an intersecting point between an extensionline of the center line of the second circular portion 108c of the guidegroove 108 extending in the direction of the arrow e and having theradius R₂ of curvature equal to the turning radius of the guide pin 117cwith the support shaft 31c as the center O₂ and an extension line of thecenter line of the straight portion 108a extending in the direction ofthe arrow b, The other intersecting point P₁ is an intersecting pointbetween the center line of the straight portion 108a and the center lineof the first circular portion 108b.

As shown in the broken lines in FIGS. 26 and 29, when the small-sizegear 124b and the guide pin 117c have reached the intersecting point P₃between the center line of the first circular portion 108b of the guidegroove 108 and the center line of the second circular portion 108c, theentire length of the disk tray 104 has been inserted in the casing 101in the direction of the arrow a, and thus the loading operation has beencompleted.

Subsequently to the above, in accordance with further positive rotationof the small-size gear 124b in the direction of the arrow m, the gearrolls along the second circular portion 109c of the rack 109 in thedirection of the arrow f. Such rolling distance corresponds to anadvancement step during overstroke operation.

Owing to the advancement step during overstroke operation of thesmall-size gear 124b in the direction of the arrow f, the gear base 117advances along the second circular portion 108c of the guide groove 108in the direction of the arrow f.

In this case, since the radius R₂ of curvature of the center line of thesecond circular portion 108c of the guide groove 108 is set to be equalto the turning radius of the guide pin 117c with the support shaft 31cas the center O₂, the guide pin 117c advances along the second circularportion 107c in the direction of the arrow f, and thus the disk tray 104remains held in the loading completion position.

Thereafter, as seen in one-dotted line of FIG. 29, through a rotationalangle of the gear base 117 or the like, a detection switch (not shown)detects that the small-size gear 124b and the guide pin 117c have beenreached the terminus P₄ of the second circular portion 108c of the guidegroove 108, the driving motor 115 is stopped.

As shown in FIGS. 25 to 27, when the small-size gear 124b advances fromthe intersecting point P₁ via the intersecting point P₃ to the terminusP₄ in the direction of the arrow f, the gear base 117 rotates about thesupport shaft 31c in the direction of the arrow f from the positionindicated in FIG. 25 to the position indicated in FIG. 27, and as aresult the cam lever 118 rotates in the direction of the arrow h fromthe position indicated in FIG. 25 to the position indicated in FIG. 27through the sectorial gear 118a meshing with the sectorial gear 117a ofthe gear base 117.

Then, the cam 120 of the cam lever 118 rotates in the direction of thearrow h with respect to the lever operating pin 110b.

Additionally, while the small-size gear 124b advances in the directionof the arrow f from the intersecting point P₁ to the intersecting pointP₃, the lever operating pin 110b of the holder driving lever 110relatively moves in the direction of the arrow g along within thelow-level portion 119b of the concave groove 119 of the cam 120, asshown in FIG. 32(A). During the advancement step of the overstrokeoperation, the small-size gear 124b advances in the direction of thearrow f from the intersecting point P₃ to the terminus P₄, the leveroperating pin 110b passes through the sloped portion 119c of the concavegroove 119 and is lifted up to the high-level portion 119a in thedirection of the arrow d, and thus the holder driving lever 110 rotatesin the direction of the arrow d.

For the reasons indicated above, during the advancement step of theoverstroke operation of the small-size gear 124b in the direction of thearrow f after completion of loading of the disk tray 104, the unitholder 40 rotates in the direction of the arrow d from the descendedposition shown in FIG. 35 to the ascended position shown in FIG. 36 byway of the driving lever 110, so as to perform a chucking operation ofthe optical disk D. The optical disk D is pushed up within the concavity105a of the disk tray 104 and thus the disk is magnetically absorbedonto the disk table 57.

Disk Tray Eject Operation

Next, the disk tray 104 is ejected out of the casing 101 in thedirection of the arrow b in the reverse order as the above-noted loadingoperation, by shifting the eject switch (not shown) to its ON state, orby rotating the driving motor 115 in its negative rotational directionin accordance with an eject signal generated from a host computer (notshown).

That is to say, as shown in one-dotted line of FIGS. 27 and 29, when thesmall-size gear 124b is rotated in its negative direction indicated bythe arrow n at the terminus P₄ of the second circular portion 108c ofthe guide groove 108 by means of the driving motor 115, the small-sizegear travels along the second circular portion 109c of the rack 109 inthe direction of the arrow e from the intersecting point P₄ to theintersecting point P₃ at the return step of the overstroke operation, asshown in FIGS. 26 and 29, while the gear base 117 rotates in thedirection of the arrow e from the position indicated in FIG. 27 to theposition indicated in FIG. 26.

For the reasons indicated above, the cam lever 118 in the direction ofthe arrow g from the position indicated in FIG. 27 to the positionindicated in FIG. 26, while the lever operating pin 110b slides from thehigh-level portion 119a of the concave groove 119 through the slopedportion 119c down to the low-level portion 119b by way of dead load asseen in FIG. 32(A), and thus the holder driving lever 110 rotates in thedirection of the arrow c by way of dead load.

In synchronization with rotational movement of the holder driving lever110, the unit holder 40 rotates in the direction of the arrow c by theaid of dead load from the descended position indicated in FIG. 36 to theascended position indicated in FIG. 35, with the result that thechucking release operation of the optical disk D is executed and theoptical disk D is placed on the concavity 105a.

When the small-size gear 124b further rotates in the reverse rotationaldirection as indicated by the arrow n, the small-size gear 124b and theguide pin 117c advance respectively along the first circular portions109b and 108b of the rack 109 and the guide groove 108 in the directionof the arrow e from the intersecting point P₃ to the intersecting pointP₁ as seen in FIGS. 26 and 29.

At this time, by the aid of cam action between the first circularportion 108b of the guide groove 108 and the guide pin 117c, i.e., by acomponent force acting in the direction of the arrow b, created bypushing the circular surface 108b₁ of the first circular portion 108b ofthe guide groove 108 through the guide pin 117c in the direction of thearrow f, the disk tray 104 is pushed out of the casing 101 by thedistance L₁ in the direction of the arrow e.

Additionally, as seen in FIG. 32(A), the lever operating pin 110brelatively moves in the direction of the arrow h within the low-levelportion 119b of the concave groove 119.

When the small-size gear 124b rotates continuously in the negativerotational direction as indicated by the arrow n, so as to drive thestraight portion 109a of the rack 109 in the direction of the arrow b,the disk tray 104 is ejected out of the casing 101 in the direction ofthe arrow b as seen in FIG. 37, and then the small-size gear 124b andthe guide pin 117c relatively move along the straight portion 108a ofthe guide groove 108 in the direction of the arrow a to the positionindicated by the two-dotted line of FIG. 29.

Thereafter, as shown in FIG. 34, through abutment of the projectedportion 105d of the disk tray 104 to the stopper 32a of the chassis 30,the ejecting operation of the disk tray 104 is completed, and then thecompletion of the eject is detected by an eject-completion switch (notshown) and the driving motor 115 is stopped.

Emergency Eject Operation

In the disk-tray loading state shown in FIG. 38, when inserting theemergency eject member 107 through the emergency eject hole 102b of thefront panel 102 in the direction of the arrow a, the top end of theemergency eject member 107 is forced onto the slider 112 provided for anemergency ejecting operation as shown in FIG. 32, and then the slider112 is pushed out in the direction of the arrow a from the positionindicated by one-dotted line of FIG. 32 to the position indicated by thesolid line by the distance L₂.

As a result, the cam lever 118 rotates in the direction of the arrow gfrom the position indicated in FIG. 27 to the position indicated in FIG.25, and thus the ejecting operation of the disk tray 104 is made, andadditionally the unit holder 40 rotates in the direction of the arrow cfrom the descended position indicated in FIG. 36 to the descendedposition indicated in FIG. 35 with the aid of dead load insynchronization with rotational movement of the holder driving lever110, in order to perform the chucking release operation of the disktable 57 to the optical disk D.

After the chucking release to the optical disk D, the guide pin 117c ofthe gear base 117 turning in the direction of the arrow e, pushes thecircular surface 108b₁ of the first circular portion 108 of the disktray 104 in the direction of the arrow f. At this time, the disk tray104 is pushed out of the casing 101 by the distance L₁ in the directionof the arrow b by the component force F₁ created by the cam action andacting in the direction of the arrow b.

Thereafter, the emergency eject operation terminates by manually pullingmore of the disk tray out of the casing 101 in the direction of thearrow b, grasping the front panel 106 of the disk tray 104, as shown inFIG. 37.

In the embodiment, since the invention is exemplified in case of anoptical disk unit for recording and reproducing an optical disk D byutilizing a disk tray 104, the concept of the invention cannot belimited to such an example. The invention can be applied to an opticaldisk unit for recording and reproducing the optical disk (not shown) ina disk cartridge (not shown).

In the embodiment, although the drive-gear mechanism 59 has a sole drivegear 59a, the invention is applied for a drive-gear mechanism having aplurality of drive gears (not shown).

Materials forming respective components in the unit according to theinvention are not limited to the materials described in the embodiment.

Moreover, in the embodiment, the invention is exemplified in case thatthe drive-gear mechanism 59 is linked to the motor gear 58a afterdriven-side transmission mechanism 62 is linked to the rack 55,according to another aspect of the invention, the driven-sidetransmission mechanism 62 may be linked to the rack 55 after linking thedrive gear 59a to the motor gear 58a.

What is claimed is:
 1. A disk unit, comprising:a motor; a drive gearmechanism having a drive gear which is driven by the motor; anintermediate transmission mechanism linked to the drive gear mechanismand having a plurality of transmission gears rotated by rotation of thedrive gear; and a head carriage having a rack in meshed engagement withat least one of the transmission gears of the intermediate transmissionmechanism and provided to be movable in a radial direction of a disk inaccordance with rotation of the transmission gears; wherein theintermediate transmission mechanism is equipped with a first backlasheliminating means for eliminating backlash between the drive gearmechanism and the intermediate transmission mechanism and with a secondbacklash eliminating means for eliminating backlash between theintermediate transmission mechanism and the rack; wherein theintermediate transmission mechanism includes a driving-side transmissionmechanism having the first backlash eliminating means and the secondbacklash eliminating means, and a driven-side transmission mechanismlinked to the driving-side transmission mechanism and to the rack;wherein the transmission gears of the driven-side transmission mechanisminclude a first driven-side transmission gear and a second driven-sidetransmission gear, the first and second driven-side transmission gearsbeing rotatable independently of each other; and wherein thetransmission gears of the driving-side transmission mechanism include afirst stepped driving-side transmission gear in meshed engagement withboth the drive gear and the second driven-side transmission gear, asecond driving-side transmission gear linked to the first driving-sidetransmission gear through the first backlash eliminating means and beingin meshed engagement with the drive gear, and a third driving-sidetransmission gear with a lever, the third driving-side transmission gearbeing rotatable attached to the second driving-side transmission gear,being linked to the first driving-side transmission gear through thesecond backlash eliminating means and being in meshed engagement withthe first driven-side transmission gear.
 2. A disk unit as claimed inclaim 1, wherein each of the first driving-side transmission gear, thesecond driving-side transmission gear and the lever has a through holefor phase matching of toothed portions of the first and seconddriving-side transmission gears.
 3. A process for assembling atransmission mechanism used for driving a head carriage, including thesteps of;linking a driven-side transmission mechanism consisting offirst and second driven-side transmission gears being rotatableindependently of each other to a rack for transmitting a driving forceproduced by a motor to a head carriage; interleaving a driving-sidetransmission mechanism between a drive gear mechanism and thedriven-side transmission mechanism after linking the drive gearmechanism with a drive gear to the motor; wherein the driving-sidetransmission mechanism includes a first stepped driving-sidetransmission mechanism being in meshed engagement with both the drivegear and the second driven-side transmission gear, a second driving-sidetransmission gear linked to the first driving-side transmission gearthrough a first compression coil spring and being in meshed engagementwith the drive gear, and a third driving-side transmission gear with alever, the third driving-side transmission gear being rotatably providedon the second driving-side transmission gear and being linked to thefirst driving-side transmission gear through a second compression coilspring and being in meshed engagement with the first driven-sidetransmission gear; inserting a support shaft portion of the thirddriving-side transmission gear into the first driving-side transmissiongear in an axial direction; rotatably linking the inserted end of thethird driving-side transmission gear to the second driving-sidetransmission gear after the inserted end is connected to the lever;resiliently disposing the first compression spring between the first andsecond driving-side transmission gears; and resiliently disposing thesecond compression spring between the driving-side transmission gear andthe lever.
 4. A disk unit comprising:a drive gear; a driving-sidetransmission mechanism including:a first driving-side transmission gearin meshed engagement with the drive gear, a second driving-sidetransmission gear in meshed engagement with the drive gear, a thirddriving-side transmission gear having a center of rotation on an axialline of the first and second driving-side transmission gears, a firstbiasing means for biasing the first and second driving-side transmissiongears so that the first driving-side transmission gear is rotationallybiased in a direction opposite to a direction of bias applied to thesecond driving-side transmission gear, and a second biasing means forbiasing the first and third driving-side transmission gears so that thefirst driving-side transmission gear is rotationally biased in adirection opposite to a direction of bias applied to the thirddriving-side transmission gear, and respective centers of rotation ofthe first, second and third driving-side transmission gears beingpositioned on an identical axial line; a driven-side transmissionmechanism having first and second driven-side transmission gears,respective centers of rotation of the first and second driven-side gearsbeing positioned on an identical axial line, the first driven-sidetransmission gear of the first and second driven-side transmission gearsbeing linked to the third driving-side transmission gear, and the seconddriven-side transmission gear being linked to the first driving-sidetransmission gear; a rack being in meshed engagement with both the firstand second driven-side transmission gears of the driven-sidetransmission mechanism; and a head carriage which moves together withthe rack.
 5. A disk unit as claimed in claim 4, wherein the firstbiasing means comprises a first coil spring connected in a compressedstate to the first driving-side transmission gear at its one end and tothe second driving-side transmission gear at its other end, and thesecond biasing means comprises a second coil spring being connected in acompressed state to the first driving-side transmission gear at its oneend and to the third driving-side transmission gear at its other end. 6.A disk unit as claimed in claim 5, wherein the third driving-sidetransmission gear comprises a gear portion being in meshed engagementwith the first driven-side transmission gear and a lever portionintegrally formed with the gear portion, and wherein the other end ofthe second coil spring is connected to the lever portion.
 7. A disk unitas claimed in claim 4, wherein the first driving-side transmission gearcomprises a first gear portion that has a diameter which is the same asa diameter of the third driving-side transmission gear and is in meshedengagement with the second driven-side transmission gear, and a secondgear portion having a same diameter as the second driving-sidetransmission gear and is in meshed engagement with the drive gear.
 8. Adisk unit as claimed in claim 4, wherein a first transmission path isformed by driving the drive gear in a first direction, the firsttransmission path being constructed by the second driving-sidetransmission gear, the first biasing means, the first driving-sidetransmission gear, and the second driven-side transmission gear, and asecond transmission path is formed by driving the drive gear in a seconddirection opposite to the first direction, the second transmission pathbeing constructed by the first driving-side transmission gear, thesecond biasing means, the third driving-side transmission gear, and thefirst driven-side transmission gear.
 9. A disk unit as claimed in claim8, wherein the first transmission path is constructed so that a drivingforce is transmitted through the second driving-side transmission gear,the first biasing means, the first driving-side transmission gear, andthe second driven-side transmissions gear in that order, and the secondtransmission path is constructed so that a driving force is transmittedthrough the first driving-side transmission gear, the second biasingmeans, the third driving-side transmission gear, and the firstdriven-side transmission gear in that order.
 10. A disk unit as claimedin claim 8, wherein the first transmission path and the secondtransmission path do not overlap.